ES 105 17 Jan 2006 Structure of the atom I. Atom is smallest particle that still retains the properties of the element A. Internal structure important to determining the properties of atoms B. Much information gathered indirectly II. Atoms have electrical properties A. Discovered in 1807 that electricity passed through molten salts released elemental reactive metal (H. Davy B. Electrolyte conducts electricity by charged atoms called IONS (M. Faraday, c. 1840): CATIONS are positive, ANIONS are negative C. Electrical current passed through low-pressure gas glowed (J. Thompson, 1897), is deflected by an electrical field showed it has negative charge, concluded ELECTRONS are parts of all atoms. Found the mass/charge ratio for electrons. D. Positive particles discovered 1886 (E. Goldstein) by putting holes in cathode of CRT, stripping atom of its electrons. Deflection in a magnetic field showed various masses of these positive particles. Hydrogen ions had the lowest mass found, which is 1837 times the mass of an electron. III. Mass of atomic particles A. Static imparted to oil drops can show the charge of a single electron (R. Millikan, 1909). And since MasseCheChMass=× argarg, Now mass of electron known: 9.1x10-28 g B. Radioactivity discovered 1896 by H. Becquerel, named by M. Sklodowska (Curie) , and three types of radioactivity determined by 1900. 1. Named ALPHA, BETA and GAMMA by E. Rutherford 2. Alpha particles shown to have 2X charge of electron, but opposite sign, and 4X mass of hydrogen atom 3. Beta particles are identical to electrons 4. Gamma rays are like X-rays, but more penetrating C. E. Rutherford bombarded metal foil with alpha particles showed most passed through, occasional particles bounced back toward source 1. Proposed tiny central portion of atom (1/10,000 of atom) contains most of the mass and a positive charge, and the rest is empty space populated by tiny electrons 2. Analogy of atomic model a. Superdome with pea in the center contains all the mass of superdome—the NUCLEUS b. A few flies within the superdome—the electrons 3. Goldstein’s smallest particle (hydrogen ion) is a PROTON D. Protons and electrons exist in equal quantities in neutral atoms1. Most atomic nuclei have masses greater than protons X # of electrons—Helium: charge +2, mass 4X proton mass ?? 2. J. Chadwick (1932) discovered neutral particle with nearly same mass as proton— a. Called it a NEUTRON b. Accounted for larger masses of atomic nuclei IV. The structure of the Nucleus A. Composed of neutrons and protons— 1. collectively called NUCLEONs 2. each as nearly the same mass 3. total number of nucleons determines ATOMIC MASS of the atom B. Number of protons is the ATOMIC NUMBER 1. controls the properties of the atom 2. Determines the element—constant for every atom of a substance C. Number of neutrons in the nucleus of an element may be different from one atom to another 1. results in some atoms of a substance having a different atomic mass than other atoms of the substance 2. these different-neutron atoms are called ISOTOPES of the element a. different isotopes of the same element have the same number of protons, but different number of neutrons b. does not change the chemical properties of the element (very much) c. does affect the nuclear properties of the atom 3. Isotope notation a. Atomic number on lower left b. Number of nucleons on upper left V. The nature of the electron arrangement of atoms A. Not randomly distributed around nucleus 1. elements give off discrete wavelengths of light when heated 2. wavelength function of energy given off when electron changes from one energy level to another— a. like a ladder b. can be on one rung or another, not in between 3. electrons return to their lowest energy level if not energized B. contained in ENERGY LEVELS (N. Bohr, 1922) 1. each energy level has a maximum number of electrons it can contain a. first can have two b. second can have up to 8 c. successive energy levels temporarily fill with 8 electrons 1) third can have 18, but often more electrons go to fourth after third has 82) fourth can have 32, but often more electrons go to fifth after fourth has 8 2. Bohr diagram showing electron configuration of an atom a. Because the electrons ‘orbit’ the nucleus, the energy levels are sometimes referred to as orbital levels b. Outermost electron is what reacts with other elements c. These are the VALENCE ELECTRONS of the atom d. Electron orbital filling has determined the structure of the periodic table VI. The periodic table show symbol and atomic number with a standard arrangement A. Vertical columns called groups 1. groups have the same number of electrons in their outermost energy level 2. elements in groups have similar chemical properties B. Horizontal rows called periods 1. periods have electrons filling the same energy levels 2. properties of elements vary periodically across a period C. Properties of certain groups 1. First column—ALKALI METALS a. Extremely reactive with water, increasingly so down the periods b. All have a single valence electron 2. Second column—ALKALINE EARTH METALS a. Moderately reactive with water, increasingly so down the periods b. All have two valence electrons 3. Next to last column—HALOGENS a. React vigorously with first and second groups—to a lesser degree down the periods b. All have seven valence electrons—nearly full energy level 4. Last column—NOBLE GASES a. Have a complete set of valence electrons—no room in their outer energy level b. Result is lack of reactivity with other elements D. Two major classes of elements: METALS and NONMETALS, shown by the diagonal line from Boron-Aluminum to Polonium-Astatine E. Many periodic tables include element name, color code for physical state at room temperature, and atomic weight. Some have other specialized information such as atomic radii, valance states, isotope lists,
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